1. Field of the Invention
The present invention relates to a mask for electron beam exposure which exposes a desired pattern on a resist film by using an electron beam, and an electron beam drawing method using the mask for electron beam exposure.
2. Description of the Related Art
In manufacturing of a semiconductor device, it is required to develop a manufacturing method by which a high through-put can be obtained. In the lithography process, in the case of forming a fine pattern on a semiconductor wafer, recently, various types of techniques to raise the through-put are also proposed. An example may be found not only in optical lithography by ultra violet light, and X-ray lithography by X-ray, but also in electron beam lithography by an electron beam. These examples utilize a transfer method using a mask for electron beam exposure. This mask for electron beam exposure has a hole of a desired pattern which has previously been formed similar to the optical or the X-ray mask and reticle.
The drawing method using a mask for electron beam exposure is generally utilized in the manufacturing of memory devices such as a DRAM (Dynamic Random Access Memory) and an SRAM (Static Random Access Memory) which have a lot of repeating patterns. On the other hand, when manufacturing logic devices such as an ASIC (Application Specific Integrated Circuit) and a micro computer which have relatively few repeating patterns, the conventional exposure by a variable shaped electron beam is applied.
FIGS. 1A to 1C are schematic views showing the conventional drawing method using a variable shaped electron beam. It should be noted that, as shown in FIG. 1A, in a variable shaped electron beam, the maximum area capable of being drawn in a single exposure, i.e., by a shot at one time, is limited, and this maximum area is made to be the maximum irradiated area 17. As shown in FIG. 1B, in a case where the desired pattern 15 is larger than the maximum irradiated area 17, then first, in order to divide the desired pattern 15 into rectangular areas smaller than the maximum irradiated area 17, rectangular division lines 16 are designed. Then, as shown in FIG. 1C, after the desired pattern 15 has been divided into rectangular patterns 18, 19, 20, and 21, the electron beam irradiation areas are matched to the rectangular patterns 18, 19, 20 and 21 by using two pieces of apertures, and the electron beam is applied to a specified area of the resist film for each rectangular pattern 18-21. Consequently, a desired pattern 15 is exposed on the resist film.
In the case where the exposure is performed by the variable shaped electron beam, it is necessary to adjust the size of the variable shaped (rectangular) electron beam in advance. The reason is that the external environment of the electron beam lowers the formation deflection of the electron beam and the electron beam exposure device, and, the stability and the repeatability of the formation lens, the object lens and the like. That is, the optical system of the electron beam and the electron beam exposure device are made to be unstable by unstable factors. Exemplary factors include the ambient temperature of the electron beam, the vibration, the magnetic field, the high voltage power supply for accelerating voltage, and the line noise or the like produced between the power source of the device and the component unit of the device. Consequently, the required dimensional accuracy cannot be obtained, and especially, dimensional variations at the exposure area in the resist film occur.
Furthermore, since the dimensional stability of the irradiation area of the electron beam is lowered with the elapse of time, usually, it is necessary to perform adjustment after every 10 to 12 hour period. Moreover, if a high dimensional accuracy is kept by adjusting the size for every device that different in design size, the desired pattern can be exposed in a stable state.
The process to pattern the resist film to the desired shape by lithography using the variable shaped electron beam will be specifically described below. FIG. 2 is a flow chart showing the conventional patterning method of the resist film using the variable shaped electron beam. In a case where the resist film formed on the surface of a wafer is patterned, then for step 31 the electron beam drawing device is adjusted in advance. At this time, the size of the irradiated area where the electron beam is applied is adjusted so that the variable shaped (rectangular) electron beam may be applied to the resist film in a specific size. Next, for step 32, a pilot drawing is performed to extract the condition of the best exposure amount for obtaining the resist pattern of a desired size. It is necessary to determine the condition of the best exposure amount, considering the difference between the design size and the size of the resist film, obtained by the etching process after the lithography process.
Furthermore, at this step 32, the alignment condition may be selected for performing the overlapping exposure to the backing film previously patterned on the desired shape. However, commonly, since the exposure can be performed by the size and the position (coordinate) of a previously determined pattern, it is not always necessary to perform this selection of the alignment condition.
Next, for step 33, by using the best exposure amount selected by the pilot drawing result of step 32, the main drawing is performed to all wafers of 1 lot, and the development is performed to pattern the resist film. After that, for step 34, the external appearance check is performed for inspecting a drawing fault, the rubbish, or the like. After that, for step 35, the backing film is etched using the resist film patterned on a specified shape as the mask. After that, as the step 36, the external appearance check is further performed, and then, the process advances to the next process.
Thus, in the conventional electron beam lithography process using the variable shaped electron beam, the steps 31 to 36 are performed for every product lot that is different in design size.
However, in the conventional process of the electron beam lithography by the variable shaped electron beam, applied at the time of the formation of the circuit of a logic device, the process (step 31) to adjust the size of the irradiated area of the variable shaped electron beam, and the process (step 32) to select the condition of the best exposure amount for obtaining a desired resist pattern size by the pilot drawing, are necessary for every product lot that is different in design size.
Furthermore, in the conventional process of the electron beam lithography, since it is necessary to consider the difference between the design size and the size of the resist film obtained by the etching process, the work to select the best condition of the exposure for every wafer becomes necessary, if the design size, and the material, structure, thickness and the like of the backing film are different. Accordingly, there is such a problem that the time necessary for the process of the electron beam lithography (TAT, i.e., turn around time) extends for several hours.
Furthermore, the irradiated area of the variable shaped electron beam may be changed by the adjusting method of two pieces of apertures and the fluctuation of the voltage of the formation lens or the like, and consequently, there is such a problem that the dimensional accuracy of the resist pattern is lowered, too. Especially, when drawing the contact pattern, the rectangular pattern designed by a fine rule is drawn by the variable shaped electron beam, and therefore, the dimensional accuracy of the obtained resist pattern is extremely degraded.
Recently, accompanied with the tendency to increase the kinds and to decrease the quantity of production of the logic devices, the reduction of the term of production (reduction of TAT) is required. Therefore, it is studied to reduce the processing steps preventing the reduction of TAT. Furthermore, accompanied with the tendency towards high integration and fineness of the devices, especially, in the contact drawing process using the minimum design rule, it is necessary to perform the drawing in such a way that the dimensional accuracy may be within .+-.10% relative to the design size, that is, not more than 0.03 .mu.m. Accordingly, when the variable shaped electron beam that contributes to the degradation of the dimensional accuracy is used, it becomes an important subject to improve the deficiency of the drawing accuracy.